Conventionally, in an automobile manufacturing process, body members and various components of the automobile have been manufactured by stacking a plurality of metal plate members and integrally coupling them by spot-joining operation such as rivet-joining and resistance spot welding. A form of coupling metal plate members by such spot-joining has become employed widely in the fields of various kinds of cars such as railroad cars and transport aircraft such as airplane, as well as in the field of structures or the like such as home electric appliances and construction materials.
Meanwhile, in Patent Document 1 and the like, as a welding method that provides a small amount of heat generation and low levels of softening and deformation during joining operation, a friction stir welding method has been proposed for joining metal members using friction heat. Such a friction stir welding method has been employed to study technology for spot joining stacked portions of a plurality of metal plate members. Thereby, different kinds of methods of friction stir spot welding have been proposed to stably obtain better joint quality and more favorable joint conditions than conventional resistance spot welding and rivet joining (see Patent Documents 2 to 4).
Any of the proposed various methods of friction stir spot welding, however, basically employs a pin-type tool (rotary tool) with a structure in which a pin-shaped hard probe is integrally disposed at a tip of a rod-shaped body of the tool. While being rotated at a high speed, the rotary tool is inserted into the stacked portions of predetermined metal plate members. Then, a shoulder portion formed at the tip of the body of the pin-type tool is pressed onto the stacked portions to generate friction heat between the shoulder portion, the probe and the stacked portions so as to cause a plastic flow of material. Thereby, a stir region is formed around the probe and spot joining of the stacked portions of the metal plate members is performed at an insertion portion for the probe. In the spot joining operation, after completion of the spot joining, the pin-type tool is extracted from the stir region (stir portion) formed in the stacked portions of the metal plate members, which leaves a recessed portion (hole) having a shape corresponding to the insertion portion of the tip of the pin-type tool including the probe. The left hole has caused problems such as liquid accumulation at a time of coating or has internally had a problem of adverse effect on joint strength (strength of a junction) between the metal plate members to be joined.
Therefore, previously in Patent Document 5, one of the inventors of the present invention has disclosed a method using a rotary tool with a double-acting structure in which a probe and a shoulder member are individually formed and independently movable in an axial direction. In this method, the probe and the shoulder member both being rotated are respectively inserted (protruded) into or abutted against the stacked portions of metal plate members to be joined to form a friction stir region in the stacked portions. After joining together the plurality of metal plate members, while extracting the probe from the friction stir region, the shoulder member is moved ahead to press against a surface of the friction stir region so as to allow peripheral material of the friction stir region to flow into a probe hole. Thereby, the probe hole is buried and the probe becomes flush with a shoulder surface of the shoulder member. Then, the rotary tool is removed from the stacked portions.
However, the friction stir spot welding using the double-acting rotary tool has a characteristic that the single rotary tool is adaptable to different thicknesses of the metal plate members to be joined and a hole to be left after removal of the probe in a joint portion formed in the friction stir region can be effectively avoided. Thereby, the method can also provide characteristics that can effectively solve the problems such as reduced joint strength and liquid accumulation during coating. However, due to the double-acting structure of the employed rotary tool, the presence of a clearance (gap) is inevitable between the probe and the shoulder member. Consequently, material of the metal plate member to be joined included in the friction stir region (joint portion) enters and adheres to the clearance. This causes intrinsic problems in which the probe and the shoulder member have difficulty in operating (moving) independently of each other and thereby repetition of friction stir spot welding operation becomes impossible.
The following are reference documents of the related art of the present invention:    Patent Document 1: U.S. Pat. No. 5,460,317    Patent Document 2: JP-A-2001-321967    Patent Document 3: U.S. Pat. No. 6,601,751 B2    Patent Document 4: JP-A-2002-120077    Patent Document 5: JP-A-2001-259863